1. Field of the Invention
The present invention relates to an oxygen sensor for use in control of air/fuel ratio in combustion engines for automobiles, for instance.
2. Related Prior Art
So far known practical oxygen sensors for detecting an oxygen concentration of exhaust gas from automobile internal combustion engines are usually of electromotive force type, oxygen (O.sub.2) pumping current type, limited current type, etc.
The oxygen sensor of electromotive force type is to detect an oxygen concentration by comparing changing potentials by an oxygen ionization reaction with a reference potential.
The oxygen sensor of O.sub.2 pumping current type is to measure an oxygen ion electromotive force generated between solid electrolytes, for example, oxygen ion-conducting, stabilized zirconia-based solid electrolytes (ZrO.sub.2 --Y.sub.2 O.sub.3, etc.), when a current is passed therebetween.
The oxygen sensor of limited current type is to measure an oxygen ion current passing through a solid electrolyte by applying a voltage thereto and limiting the thus generated oxygen ion current by a diffusion-resistant layer.
As shown in FIG. 8, the oxygen sensor of limited current type has a sensor element 90 at its tip end. The sensor element 90 is a cup-formed element, as shown in FIGS. 8 and 9, which is formed by laminating an inner electrode 32, a ZrO.sub.2 solid electrolyte 5, an outer electrode 31 and a diffusion-resistant layer 2 successively from the inside outwardly. A heater 6 is inserted into the inner cavity 901 of the sensor element 90. An insulating layer 4 is provided between the outer periphery of the solid electrolyte 5 and the diffusion-resistant layer 2 except the site for the outer electrode 31.
The outer electrode 31 and the inner electrode 32 are connected to a connector 98 above the sensor element 90 through lead wires 91 and 92, respectively. These two electrodes are porous platinum electrodes, or the like. The heater 6 is connected to the connector 98 through a lead wire 93.
The insulating layer 4 is made of an insulator and sets an electrode area, thereby controlling an output current density. The diffusion-resistant layer 2 serves to protect the outer electrode and also control a limited current.
The sensor element 90 is fixed to an exhaust gas pipe, etc. by a flange 97 provided at a housing 96. A protective cover 95 is provided at the outside of the sensor element 90.
When a voltage is applied between the outer electrode 31 and the inner electrode 32 in the oxygen sensor 9, electro-chemical reactions take place between these two electrodes, and an oxygen concentration can be determined by detecting a current passing therebetween due to the reactions. Relations between the applied voltage and the output current are shown in FIG. 11.
The electrochemical reactions proceed while transferring electrons between the cathode (outer electrode) and the anode (inner electrode) by oxygen, as shown in FIG. 12A. That is, as shown in FIGS. 12B, 12C and 13, oxygen molecules (O.sub.2) contained in a gas phase are adsorbed on the three-phase boundary points 1 between the outer electrode 31, the solid electrolyte 5 and the gas phase.
The oxygen molecules (O.sub.2) adsorbed on the three-phase boundary points 1 are dissociated into oxygen atoms (O). The dissociated oxygen atoms (O) receive electrons (e.sup.-) from the outer electrode 31 and are ionized, while leaving the three-phase boundary points 1 as oxygen ions (O.sub.2-). The oxygen ions (O.sub.2-) migrate through the solid electrolyte 5, as shown in FIG. 12B, and reach the three-phase boundary points between the inner electrode 32, the gas phase and the solid electrolyte 5, where the oxygen ions (O.sub.2-) give electrons (e.sup.-) to the inner electrode 32. The electrochemical reactions take place in this manner.
The rate-determining step in the electrochemical reactions is an adsorption reaction of oxygen molecules onto the three-phase boundary points 1. To facilitate the adsorption it would be possible to make the outer electrode 31 porous, thereby increasing the number of the three-phase boundary points 1. However, as shown in FIG. 14, even if the number of the three-phase boundary points is increased, there is no change in the volume each of the individual three-phase boundary points 1. Furthermore, the ternary phase boundary points 1 are formed only on the surface of the solid electrolyte 5, and thus there is a limit to the available number of the three-phase boundary points.
To practically keep the oxygen sensor operative, both electrodes must be heated to a high temperature such as 700.degree. C. by the heater 6. Heat loss is large due to the working at such a high temperature, resulting in large power consumption of the heater 6.